The present invention relates to an image-pickup apparatus having an autofocus (AF) correction function in accordance with the color of an object (for example, the type of light source).
There are many cases in which image-pickup apparatuses such as single-lens reflex cameras and the like employ a focus detection method called a TTL phase difference detection method. In a TTL phase difference detection method, the light beam from an image-pickup optical system is divided into two by an optical system for phase difference detection, and two images are formed on a pair of light-receiving element lines by performing secondary image formation of these divided light beams. The defocus amount of the image-pickup optical system is then acquired by detecting the relative positional difference (phase difference) of the two images.
Generally, in an image-pickup optical system or an optical system for phase difference detection, the correction of various aberrations such as chromatic aberrations and the like is performed in the visible wavelength range from 400 nm to 650 nm centered on the d-line (587 nm). Therefore, the aberrations in the wavelength area outside of the visible wavelength range, for example, in the near-infrared wavelength range, is very rarely corrected well. In this case, since the relative ratios of the near-infrared light relative to the visible light are different in image-pickup in daylight, in a light source with a low color temperature such as tungsten lamps, and in a light source with a high color temperature such as fluorescent lamps and the like, results in which different phase differences are detected are often acquired.
On the other hand, silicon photodiodes for performing photoelectric conversion used in image-pickup elements such as CMOS sensors and the like generally have a sensitivity peak of around 800 nm, and the long wavelength side has a sensitivity of around 1,100 nm. However, in order to emphasize the color reproductivity, the sensitivity is sacrificed and light outside the aforementioned visible wavelength range is blocked by filters and the like.
Further, a photoelectric conversion element (a light-receiving element) constituting a light-receiving sensor for phase difference detection similarly has a sensitivity of around 1,100 nm. However, in consideration of performing satisfactory focus control on even low-luminance objects and irradiating near-infrared (around 700 nm) assist light on an object in a low-luminance environment, it is designed to have a sensitivity of around 100 nm more than the image-pickup optical system to include the long wavelength range.
Furthermore, the amount of chromatic aberration of an optical system changes depending on the wavelength, and the focus position also changes in accordance with this. Also, the longer the wavelength, the greater the chromatic aberration, i.e. amount of focus deviation.
Therefore, in a light-receiving sensor for phase difference detection having a maximum sensitivity in the wavelength area around 700 nm, for example, when a fluorescent lamp with a few long wavelength components is set as the light source and when a flood lamp with many long wavelength components is set as the light source, the focus positions are different even if the object distance is the same. In other words, the focus position found by the phase difference detection method must be corrected according to the type of light source (in other words, color of the object).
A method for correcting the focal point position in accordance with the type of light source (color of the object) in this manner is disclosed in Japanese Patent Laid-Open No. 2000-275512 and Japanese Patent Laid-Open No. 9-211522. The correction method disclosed in Japanese Patent Laid-Open No. 2000-275512 uses the difference in spectral sensitivity characteristics between a light-receiving sensor for phase difference detection (hereinafter referred to as an AF sensor) and a sensor for light source detection to determine the type of light source. The light source information is obtained from the output of the light source detecting sensor after the output from the AF sensor is acquired, and light source correction processing of the defocus amount is performed based on the light source information.
Further, in the correction method disclosed in Japanese Patent Laid-Open No. 9-211522, when image-pickup of the second image or beyond is performed in continuous image-pickup (continuous image-pickup), the light source correction processing of the defocus amount is performed based on the light source information detected in the image-pickup preparation operation of the first image.
However, as in the correction method disclosed in Japanese Patent Laid-Open No. 2000-275512, when the light source information is found from the output of the light source detecting sensor after the output from the AF sensor is acquired, if the acquisition of the light source information is delayed, the AF processing (focus control) is delayed. Therefore, the timing of the image-pickup operation for acquiring the image for recording is also delayed. More particularly, a delay in the acquisition of the light source information when performing continuous image-pickup obstructs the improvement of the continuous image-pickup speed.
On the other hand, as in the correction method disclosed in Japanese Patent Laid-Open No. 9-211522, using the light source information A from the first image for the image-pickup for the second image or beyond as well improves the continuous image-pickup speed. However, if the color of the object changes during continuous image-pickup, the focus may deviate in the image-pickup from the second image or beyond.